Flexible Resistive Gas Sensor Based on Molybdenum Disulfide-Modified Polypyrrole for Trace NO2 Detection

High sensitivity and selectivity and short response and recovery times are important for practical conductive polymer gas sensors. However, poor stability, poor selectivity, and long response times significantly limit the applicability of single-phase conducting polymers, such as polypyrrole (PPy). In this study, PPy/MoS2 composite films were prepared via chemical polymerization and mechanical blending, and flexible thin-film resistive NO2 sensors consisting of copper heating, fluorene polyester insulating, and PPy/MoS2 sensing layers with a silver fork finger electrode were fabricated on a flexible polyimide substrate using a flexible electronic printer. The PPy/MoS2 composite films were characterized using X-ray diffraction, Fourier-transform infrared spectroscopy, and field-emission scanning electron microscopy. A home-built gas sensing test platform was built to determine the resistance changes in the composite thin-film sensor with temperature and gas concentration. The PPy/MoS2 sensor exhibited better sensitivity, selectivity, and stability than a pure PPy sensor. Its response to 50 ppm NO2 was 38% at 150 °C, i.e., 26% higher than that of the pure PPy sensor, and its selectivity and stability were also higher. The greater sensitivity was attributed to p–n heterojunction formation after MoS2 doping and more gas adsorption sites. Thus, PPy/MoS2 composite film sensors have good application prospects.


Introduction
With the rapid development of modern industry, environmental pollution has increased significantly, and large volumes of toxic and harmful gasses are continuously released into the environment that sustains our existence, affecting the air quality and causing various problems [1][2][3][4][5].To prevent these gaseous emissions from harming humans and the environment, toxic gas monitoring is becoming increasingly important.Among the toxic gasses that require monitoring, nitrogen dioxide (NO 2 ) is a common toxic air pollutant.The main sources of NO 2 are the combustion of certain fuels, such as coal and petroleum, biomass combustion triggered by the extreme heat of lightning during thunderstorms, and the microbial fixation of nitrogen in agriculture [6][7][8][9].The major effects of NO 2 include inflammation of the respiratory tract, decreased lung function, increased reactivity to allergens, and the formation of fine particulate matter (PM) and ground-level ozone-which have adverse health effects-as well as the formation of acid rain, which damages vegetation and results in the acidification of buildings, lakes, and streams [10][11][12].The lethal concentration of NO 2 for 50% of people who are exposed (LC50) for 1 h of Polymers 2024, 16,1940 2 of 13 exposure is approximately 174 ppm; therefore, it is important to develop sensors for the detection of nitrogen dioxide with low detection limits and high sensitivity [13,14].
In recent years, conductive polymers have frequently been used as sensing materials because of their excellent physicochemical properties.They are versatile, inexpensive, and easy to synthesize and have large surface areas, low operating temperatures, and high sensitivities.In addition, exposure to various gasses leads to changes in the electrical and optical properties of these sensing materials in addition to other properties [15].Among the various types of conductive polymers, polypyrrole (PPy) has become a major focus of recent research by virtue of its chemical stability under atmospheric conditions, its ease of adhering to various substrates, and ease of synthesis by electrochemical and chemical methods.However, compared to inorganic semiconductor sensors, sensors made with PPy are less sensitive to target gasses and have longer response times, and these drawbacks limit the applicability of this material [16].Therefore, it is usually necessary to optimize the properties of PPy by doping [17].In recent years, research has shown that the mutual doping of conducting polymers and semiconducting metal oxides [18] can produce synergistic effects, with the enhancement of the performance of both materials owing to interfacial electron transfer and catalytic effects, among others.Zhang et al. prepared PPy-coated SnO 2 hollow sphere hybrid materials via in situ polymerization using SnO 2 hollow spheres as carriers, and the performance of the resulting composites was shown to be much better than that of pure PPy.The hybrid material exhibited a faster response and higher sensitivity to ammonia at room temperature than pure PPy [19].Similarly, Mane et al. fabricated PPy/WO 3 hybrid nanomaterials on glass substrates for chemoresistive NO 2 sensors using the drop-casting method [20].For the p-type semiconductor PPy, doping with n-type materials such as WO 3 can generate more p-n heterostructures, and therefore, the sensor's performance is improved.
Compared to conventional metal oxide materials, metal sulfides have unique twodimensional (2D) atomic structures, which offer significantly increased areas of contact between the gas molecules and material surface, resulting in a greater number of active sites [21,22].In addition, metal sulfides have higher charge mobilities, which allows carriers to migrate more easily inside the material and effectively extends the hole and electron lifetimes [23].Among the many metal sulfides available, MoS 2 is a typical 2D transition metal sulfide with a graphene-like layered structure that has been widely investigated as a promising candidate for various applications owing to its excellent properties [24].Han et al. constructed a NO 2 sensor with a unique 2D/0D MoS 2 /SnO 2 p-n heterostructure based on MoS 2 nanosheets on a Si/SiO 2 substrate, which exhibited a high response value of 18.7 at a low NO 2 concentration (5 ppm) [25].Similarly, Liu et al. designed and fabricated a composite material based on MoS 2 nanomaterials modified with tin disulfide (SnS 2 ) by mechanical exfoliation combined with the hydrothermal method; the mechanically exfoliated MoS 2 nanosheets acted as the central sensing channel, interacting with gaseous NO 2 , while the SnS 2 nanoparticles decorating the nanosheet surfaces acted as a surface passivation layer to prevent the oxidation of the MoS 2 nanosheets.Owing to the synergistic effect of the excellent gas sensing properties of the MoS 2 nanosheets and the surface-protecting properties of the SnS 2 nanoparticles, the prepared MoS 2 /SnS 2 composites exhibited reliable long-term stability with a response/recovery rate of 28s/3s for the sensing of NO 2 [26].Ding et al. synthesized MoS 2 /rGO/Cu 2 O ternary composites on an alumina ceramic substrate using the soft stenciling method, and gas sensing tests showed a good response (14.88%) to NO 2 for the MoS 2 /rGO/Cu 2 O composite sensor at the ppb level at room temperature.This good response is attributed to the positive synergistic effect of the three-phase composite material, as the gas permeation and diffusion channels are tightly loaded on MoS 2 /rGO nanosheets, giving the composite material a larger surface area [27].
However, in recent years, research on NO 2 sensors has mostly been focused on the use of glass, SiO 2 , and other rigid substrate materials.Such sensors have poor mechanical flexibility, are easily damaged by mechanical impact, and have high coefficients of thermal expansion and performances that are more strongly dependent on temperature [28]; in addition, they cannot be tightly fitted onto uneven or complex surfaces.In contrast, the covalent bonding forces are very strong within metal sulfide layers, so these materials have good mechanical properties.Therefore, flexible and wearable sensors based on PPy/MoS 2 composites have excellent potential application prospects and should therefore be developed.
In this work, we synthesized a new PPy/MoS 2 binary composite material by chemical polymerization and mechanical blending using a flexible electronic printer, dispensing printed copper micro-heaters with silver fork finger electrodes.PPy/MoS 2 composites were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR), and the structural morphology of the composites was studied in detail.A flexible thin-film NO 2 sensor was then prepared by coating the material onto a silver fork finger electrode on a polyimide (PI) substrate via dispensing printing.A gas sensing test platform was built to sense gas concentrations between 1 and 100 ppm.Finally, the gas sensing mechanism was investigated, and the improved sensing performance was understood.

Characterization Methods
The structures of PPy and PPy/MoS 2 were analyzed using an X-ray diffractometer (Rigaku Smartlab 3kw, Tokyo, Japan) under Cu Kα radiation in the scanning range of 20-80 • .The valence states of the elements were analyzed by X-ray photoelectron spectroscopy (Thermo ESCALAB 250Xi, Los Angeles, CA, USA).The morphology of the samples was observed by field-emission scanning electron microscopy (ZEISS Gemini SEM 300, Berlin, Germany) at an accelerating voltage of 3-15 kV.FTIR spectroscopy (Perkin Elmer 100, Los Angeles, CA, USA) was used for the chemical structure analysis within the frequency range of 400-4000 cm −1 .

Preparation of Sensing Material Composites
First, PPy powder was prepared via chemical oxidative polymerization using pyrrole as the monomer and ammonium persulfate (APS) as the oxidant.First, pyrrole (0.7 mL) and APS powder (2.28 g) were separately dissolved in diluted hydrochloric acid (1 M, 100 mL), and then an acidic APS solution was added to an acidic pyrrole solution at the rate of 3 drops per second, and they were mixed together in a beaker for 4 h to allow the polymerization reaction to proceed.After polymerization, the precipitate obtained after vacuum filtration was washed six times with methanol and deionized water and then dried in a vacuum heating chamber at 60 • C for 12 h to obtain PPy powder [29].
PPy/MoS 2 composites were prepared using the mechanical blending method.MoS 2 and PPy powders were added to a smooth agate mortar in a specific ratio, ground with a pestle and mortar for 3 h, dispersed in dimethylformamide (DMF) solution (5 mL) with magnetic stirring for 3 h, and finally dried at 80 • C for 24 h in a heating chamber to obtain the PPy/MoS 2 composite as a homogeneous mixture (Figure 1).

Sensor Structure Design and Gas Sensing Test Platform
The sensor was fabricated on a PI substrate using a flexible electronic printer (Scientific 3, Zhongbin Technology) to perform dispensing printing.The flexible thin-film sensor substrate consisted of a carbon heating layer, an insulating layer, a silver fork finger electrode, and a sensing layer (Figure 2a,b).The insulating layer was made from FPE via the following method: FPE (0.4 g) was dissolved in N-methyl-2-pyrrolidone (NMP) (4 mL), and the resulting solution was stirred rigorously for 2 h before being scraped and coated onto the heating layer using a flexible printer and then dried at 80 °C for 24 h.In our previous studies [30], FPE was demonstrated to be a high-performance insulation material with a high dielectric constant (3.5) and a glass transition temperature of 335 °C, which allows it to perform well as a sensor insulation layer.The dried powder sample obtained as the PPy/MoS2 composite (Section 2.3) was ground to a 200 mesh size in an agate mortar and sieved.An appropriate amount of rosinol was added dropwise to the milled composite material, which was then coated onto the silver fork finger electrode layer of the sensor via dispensing printing with a flexible electronic printer.The fabricated sensor was vacuum-dried at 60 °C for 12 h and aged at its operating temperature for three days.The gas sensing test platform consisted of a gas dispenser, DC power supply, data acquisition card, and PC (Figure 2c).An amount of gas was injected into the gas chamber (1 L) using a gas dispenser to achieve a specific final concentration of the gas.A DC power supply provided the operating voltage for the carbon heater in the sensor.The adsorption of the gas onto the gas sensing material resulted in a change in the sensor resistance, and the data acquisition card recorded the change in resistance and uploaded it to the PC to record the data.The sensor film photo is shown in Supplementary Document S1.

Sensor Structure Design and Gas Sensing Test Platform
The sensor was fabricated on a PI substrate using a flexible electronic printer (Scientific 3, Zhongbin Technology, Harbin, China) to perform dispensing printing.The flexible thin-film sensor substrate consisted of a carbon heating layer, an insulating layer, a silver fork finger electrode, and a sensing layer (Figure 2a,b).The insulating layer was made from FPE via the following method: FPE (0.4 g) was dissolved in N-methyl-2-pyrrolidone (NMP) (4 mL), and the resulting solution was stirred rigorously for 2 h before being scraped and coated onto the heating layer using a flexible printer and then dried at 80 • C for 24 h.In our previous studies [30], FPE was demonstrated to be a high-performance insulation material with a high dielectric constant (3.5) and a glass transition temperature of 335 • C, which allows it to perform well as a sensor insulation layer.The dried powder sample obtained as the PPy/MoS 2 composite (Section 2.3) was ground to a 200 mesh size in an agate mortar and sieved.An appropriate amount of rosinol was added dropwise to the milled composite material, which was then coated onto the silver fork finger electrode layer of the sensor via dispensing printing with a flexible electronic printer.The fabricated sensor was vacuum-dried at 60 • C for 12 h and aged at its operating temperature for three days.The gas sensing test platform consisted of a gas dispenser, DC power supply, data acquisition card, and PC (Figure 2c).An amount of gas was injected into the gas chamber (1 L) using a gas dispenser to achieve a specific final concentration of the gas.A DC power supply provided the operating voltage for the carbon heater in the sensor.The adsorption of the gas onto the gas sensing material resulted in a change in the sensor resistance, and the data acquisition card recorded the change in resistance and uploaded it to the PC to record the data.The sensor film photo is shown in Supplementary Figure S1.

Sensor Structure Design and Gas Sensing Test Platform
The sensor was fabricated on a PI substrate using a flexible electronic printer (Scientific 3, Zhongbin Technology) to perform dispensing printing.The flexible thin-film sensor substrate consisted of a carbon heating layer, an insulating layer, a silver fork finger electrode, and a sensing layer (Figure 2a,b).The insulating layer was made from FPE via the following method: FPE (0.4 g) was dissolved in N-methyl-2-pyrrolidone (NMP) (4 mL), and the resulting solution was stirred rigorously for 2 h before being scraped and coated onto the heating layer using a flexible printer and then dried at 80 °C for 24 h.In our previous studies [30], FPE was demonstrated to be a high-performance insulation material with a high dielectric constant (3.5) and a glass transition temperature of 335 °C, which allows it to perform well as a sensor insulation layer.The dried powder sample obtained as the PPy/MoS2 composite (Section 2.3) was ground to a 200 mesh size in an agate mortar and sieved.An appropriate amount of rosinol was added dropwise to the milled composite material, which was then coated onto the silver fork finger electrode layer of the sensor via dispensing printing with a flexible electronic printer.The fabricated sensor was vacuum-dried at 60 °C for 12 h and aged at its operating temperature for three days.The gas sensing test platform consisted of a gas dispenser, DC power supply, data acquisition card, and PC (Figure 2c).An amount of gas was injected into the gas chamber (1 L) using a gas dispenser to achieve a specific final concentration of the gas.A DC power supply provided the operating voltage for the carbon heater in the sensor.The adsorption of the gas onto the gas sensing material resulted in a change in the sensor resistance, and the data acquisition card recorded the change in resistance and uploaded it to the PC to record the data.The sensor film photo is shown in Supplementary Document S1.

Results and Discussion
Figure 3a shows an SEM image of the prepared PPy, and Figure 3b shows a higherresolution image from the same field of view.The PPy sample exhibits spherical particle stacking [31] with a particle diameter of approximately 100 nm.Most of the PPy particles are uniformly distributed, and the material as a whole has porous characteristics, suggesting that gas molecules will be easily adsorbed by the sensing material.The characteristic C and O peaks are shown in Supplementary Figure S2, and the weight percentages of elemental C and O were calculated to be as high as 6.55, indicating that the pyrrole monomer was fully polymerized and oxidized [32].SEM images of the PPy/MoS 2 samples are shown in Figure 3c-f.The structure was assembled from the PPy spherical structure and the 2D lamellar structure of MoS 2 .It can be seen that MoS 2 is well embedded within the PPy, and a small amount of PPy is directly adhered to the MoS 2 sheets.The distributions of C, O, Mo, and S are shown in Figure 3(f1)-(f4), which reveal that the MoS 2 is uniformly distributed among the C and O distributions of PPy, indicating a more homogeneous distribution of PPy.In Figure 3, it can be seen that doping with the 2D MoS 2 sheets produces many porous structures, providing additional adsorption sites in the material.According to the cross-sectional analysis in Supplementary Figure S3, it can be seen that the composite sensing membrane is approximately 2 µm thick, and the introduction of MoS 2 nanosheets adds a large number of adsorption sites to the composite material, suggesting that the gas-sensitive performance of the composite material might be superior to that of pure PPy.

Results and Discussion
Figure 3a shows an SEM image of the prepared PPy, and Figure 3b shows a higherresolution image from the same field of view.The PPy sample exhibits spherical particle stacking [31] with a particle diameter of approximately 100 nm.Most of the PPy particles are uniformly distributed, and the material as a whole has porous characteristics, suggesting that gas molecules will be easily adsorbed by the sensing material.The characteristic C and O peaks are shown in Supplementary Document S2, and the weight percentages of elemental C and O were calculated to be as high as 6.55, indicating that the pyrrole monomer was fully polymerized and oxidized [32].SEM images of the PPy/MoS2 samples are shown in Figure 3c-f.The structure was assembled from the PPy spherical structure and the 2D lamellar structure of MoS2.It can be seen that MoS2 is well embedded within the PPy, and a small amount of PPy is directly adhered to the MoS2 sheets.The distributions of C, O, Mo, and S are shown in Figure 3f1-f4, which reveal that the MoS2 is uniformly distributed among the C and O distributions of PPy, indicating a more homogeneous distribution of PPy.In Figure 3, it can be seen that doping with the 2D MoS2 sheets produces many porous structures, providing additional adsorption sites in the material.According to the cross-sectional analysis in Supplementary Document S3, it can be seen that the composite sensing membrane is approximately 2 µm thick, and the introduction of MoS2 nanosheets adds a large number of adsorption sites to the composite material, suggesting that the gas-sensitive performance of the composite material might be superior to that of pure PPy. Figure 4a shows the XRD patterns of the pure PPy and various PPy/MoS2 samples at 25 °C.It can be seen that the diffraction peaks of almost all of the binary materials are in good agreement with those of the pure PPy material, and there is a broad peak between 2θ = 20 and 30°, indicating that the polymer is amorphous [33].The broad peaks were caused by the X-ray scattering of the PPy chains.The presence of these peaks in each spectrum indicates that the introduction of MoS2 did not disrupt the structure of PPy itself.The diffraction peaks at 2θ = 32.6,39.55, 44.3, and 59° are attributed to the (100), ( 103), (006), and (105) planes of MoS2, respectively [34].As the MoS2 mass fraction increased, the Figure 4a shows the XRD patterns of the pure PPy and various PPy/MoS 2 samples at 25 • C. It can be seen that the diffraction peaks of almost all of the binary materials are in good agreement with those of the pure PPy material, and there is a broad peak between 2θ = 20 and 30 • , indicating that the polymer is amorphous [33].The broad peaks were caused by the X-ray scattering of the PPy chains.The presence of these peaks in each spectrum indicates that the introduction of MoS 2 did not disrupt the structure of PPy itself.The diffraction peaks at 2θ = 32.6,39.55, 44.3, and 59 • are attributed to the (100), ( 103), (006), and (105) planes of MoS 2 , respectively [34].As the MoS 2 mass fraction increased, the characteristic peaks assigned to this compound became pronounced, indicating that Polymers 2024, 16, 1940 6 of 13 the MoS 2 sheets and PPy were well mixed.Figure 4b shows the FTIR spectra of the two samples, where characteristic absorption bands are observed at 1550, 1451, 1181, 1046, 1046, and 794 cm −1 ; these were attributed to the C-C vibration in the pyrrole ring, the C-N respiratory vibrations of the pyrrole ring, and the C-N and N-H in-plane deformation vibrations, respectively [35,36].The four composites exhibit absorption bands close to those of PPy.All of the above observed absorption properties confirm the synthesis of PPy and that its functional groups are not destroyed when it is doped with MoS 2 .
Polymers 2024, 16, x FOR PEER REVIEW characteristic peaks assigned to this compound became pronounced, indicating MoS2 sheets and PPy were well mixed.Figure 4b shows the FTIR spectra of the t ples, where characteristic absorption bands are observed at 1550, 1451, 1181, 104 and 794 cm −1 ; these were attributed to the C-C vibration in the pyrrole ring, the C piratory vibrations of the pyrrole ring, and the C-N and N-H in-plane deformatio tions, respectively [35,36].The four composites exhibit absorption bands close to PPy.All of the above observed absorption properties confirm the synthesis of that its functional groups are not destroyed when it is doped with MoS2.To determine a suitable working temperature for the flexible gas sensor, w ured the thermal decomposition temperatures of both PPy and PPy/MoS2 (10%) ZCT−B thermogravimetric (TG) analyzer, as shown in Figure 5.The TG and diff thermal analysis (DTA) curves were obtained at a scanning rate of 10 °C/min.Th in Figure 5 are the TG and DTA curves, respectively.The PPy sample continuo weight during heating from the start (at 100 °C).The thermal decomposition of be divided into two stages: (1) the first stage of weight loss occurred at the range 420 °C and resulted from the detachment of water molecules from PPy; (2) the we between 420 and 650 °C was a result of the degradation of the PPy macromolecula The polymer completely decomposed above 650 °C.The TG and DTA curves re constant at temperatures above 650 °C, which indicates that the decomposition began at 300 °C [37].Figure 5b shows the TG and DTA curves of the PPy/MoS2 com These have similar shapes to those of PPy, with two relatively symmetric and dist absorption peaks at the ranges of 350−400 and 400−530 °C.The composite mate very little weight below 250 °C.When compared with that of pure PPy, the ther bility of PPy in the hybrid material is almost unchanged, and the TG analysis i that that the maximum operating temperature of the sensor can be set to 350 °C.To determine a suitable working temperature for the flexible gas sensor, we measured the thermal decomposition temperatures of both PPy and PPy/MoS 2 (10%) using a ZCT-B thermogravimetric (TG) analyzer, as shown in Figure 5.The TG and differential thermal analysis (DTA) curves were obtained at a scanning rate of 10 • C/min.The curves in Figure 5  characteristic peaks assigned to this compound became pronounced, indicating tha MoS2 sheets and PPy were well mixed.Figure 4b shows the FTIR spectra of the two ples, where characteristic absorption bands are observed at 1550, 1451, 1181, 1046, and 794 cm −1 ; these were attributed to the C-C vibration in the pyrrole ring, the C-N piratory vibrations of the pyrrole ring, and the C-N and N-H in-plane deformation v tions, respectively [35,36].The four composites exhibit absorption bands close to tho PPy.All of the above observed absorption properties confirm the synthesis of PPy that its functional groups are not destroyed when it is doped with MoS2.To determine a suitable working temperature for the flexible gas sensor, we m ured the thermal decomposition temperatures of both PPy and PPy/MoS2 (10%) us ZCT−B thermogravimetric (TG) analyzer, as shown in Figure 5.The TG and differe thermal analysis (DTA) curves were obtained at a scanning rate of 10 °C/min.The cu in Figure 5 are the TG and DTA curves, respectively.The PPy sample continuously weight during heating from the start (at 100 °C).The thermal decomposition of PPy be divided into two stages: (1) the first stage of weight loss occurred at the range of 420 °C and resulted from the detachment of water molecules from PPy; (2) the weigh between 420 and 650 °C was a result of the degradation of the PPy macromolecular ch The polymer completely decomposed above 650 °C.The TG and DTA curves rema constant at temperatures above 650 °C, which indicates that the decomposition of began at 300 °C [37].Figure 5b shows the TG and DTA curves of the PPy/MoS2 compo These have similar shapes to those of PPy, with two relatively symmetric and distinct absorption peaks at the ranges of 350−400 and 400−530 °C.The composite materia very little weight below 250 °C.When compared with that of pure PPy, the therma bility of PPy in the hybrid material is almost unchanged, and the TG analysis indi that that the maximum operating temperature of the sensor can be set to 350 °C.To investigate the effect of the MoS 2 content on the gas sensing performance of the PPy-based NO 2 sensors and impact resistance, five composite sensors with different MoS 2 mass fractions (0-30%) were tested using 100 ppm NO 2 at room temperature to observe the response.The highest response (49.8%) was obtained for the sensor made with the composite containing 10% MoS 2 , and the lowest response was obtained for that made with the 30% MoS 2 composite.Figure 6a shows the changes in the resistance values of the different sensors after the chamber of the gas sensing test platform was charged with gas.It is apparent that as the MoS 2 content increased, the resistance of the composite material decreased.In this study, the sensitivity (response) of the sensor was defined in terms of the resistance of the gas sensor in air R a and in the sample gas R g as follows: As shown in Figure 6b, the PPy/MoS 2 sensor made with the 10% MoS 2 composite has the highest sensitivity (49.8%) to 100 ppm NO 2 at room temperature.However, Figure 6a shows that the response/recovery times of several of the sensors at room temperature are long, which is unfavorable in practical applications.To optimize the performance of the sensors, the 10% PPy/MoS 2 composite was selected for subsequent experiments in which the temperature was increased in an attempt to minimize the response/recovery time of the sensor.
Polymers 2024, 16, x FOR PEER REVIEW 7 To investigate the effect of the MoS2 content on the gas sensing performance of PPy−based NO2 sensors and impact resistance, five composite sensors with different M mass fractions (0−30%) were tested using 100 ppm NO2 at room temperature to obs the response.The highest response (49.8%) was obtained for the sensor made with composite containing 10% MoS2, and the lowest response was obtained for that made w the 30% MoS2 composite.Figure 6a shows the changes in the resistance values of the ferent sensors after the chamber of the gas sensing test platform was charged with ga is apparent that as the MoS2 content increased, the resistance of the composite mat decreased.In this study, the sensitivity (response) of the sensor was defined in term the resistance of the gas sensor in air Ra and in the sample gas Rg as follows: As shown in Figure 6b, the PPy/MoS2 sensor made with the 10% MoS2 composite the highest sensitivity (49.8%) to 100 ppm NO2 at room temperature.However, Figur shows that the response/recovery times of several of the sensors at room temperature long, which is unfavorable in practical applications.To optimize the performance of sensors, the 10% PPy/MoS2 composite was selected for subsequent experiments in w the temperature was increased in an attempt to minimize the response/recovery tim the sensor.As shown in Figure 7, the sensitivity of the sensor decreased as the temperatur creased.By the time the temperature reached 150 °C, during a continuous increase in t perature, the sensitivity of the sensor reduced from 49.89% to 38.75%, and its respo and recovery times decreased to 120 and 57 s, respectively; at room temperature, th sponse and recovery times were 572 and 461 s, respectively.Thus, it was demonstr that at 150 °C, although the sensitivity of the sensor was slightly reduced, the efficienc gas adsorption and desorption was greatly improved, and hence, the recovery time dramatically reduced.Therefore, 150 °C can be considered a good working tempera for the sensor.As shown in Figure 7, the sensitivity of the sensor decreased as the temperature increased.By the time the temperature reached 150 • C, during a continuous increase in temperature, the sensitivity of the sensor reduced from 49.89% to 38.75%, and its response and recovery times decreased to 120 and 57 s, respectively; at room temperature, the response and recovery times were 572 and 461 s, respectively.Thus, it was demonstrated that at 150 • C, although the sensitivity of the sensor was slightly reduced, the efficiency of gas adsorption and desorption was greatly improved, and hence, the recovery time was dramatically reduced.Therefore, 150 • C can be considered a good working temperature for the sensor.
In order to investigate the performance of the sensors at low NO 2 concentrations, the resistances of the pure PPy and 10% PPy/MoS 2 composite sensors at 150 • were measured under a concentration gradient change from 1 to 20 ppm (Figure 8).From the response curves, it can be seen that the resistance of the sensor decreased significantly as the gas concentration increased; at 1 ppm, the sensitivity of the pure PPy sensor was 2.7%, and that of the PPy/MoS 2 sensor was 8%, while the response and recovery times of the latter were shorter.Therefore, the sensor has good resolution even at low NO 2 gas concentrations, and it can provide a basis for the development of improved trace gas detection techniques.
sponse and recovery times were 572 and 461 s, respectively.Thus, it was d that at 150 °C, although the sensitivity of the sensor was slightly reduced, the gas adsorption and desorption was greatly improved, and hence, the recov dramatically reduced.Therefore, 150 °C can be considered a good working for the sensor.In order to investigate the performance of the sensors at low NO2 concentrations, the resistances of the pure PPy and 10% PPy/MoS2 composite sensors at 150° were measured under a concentration gradient change from 1 to 20 ppm (Figure 8).From the response curves, it can be seen that the resistance of the sensor decreased significantly as the gas concentration increased; at 1 ppm, the sensitivity of the pure PPy sensor was 2.7%, and that of the PPy/MoS2 sensor was 8%, while the response and recovery times of the latter were shorter.Therefore, the sensor has good resolution even at low NO2 gas concentrations, and it can provide a basis for the development of improved trace gas detection techniques.As shown in Table 1, the PPy/MoS2 composite thin-film sensor prepared in this work has a lower operating temperature than traditional metal oxide sensors, and it has a faster response/recovery time than pure conductive polymer thin-film sensors.Figure 9a shows the repeatability of the PPy/MoS2 sensor measurements for NO2 gas at 100 ppm, where it is apparent that the response and recovery characteristics were maintained without significant degradation.Over six cycles of measurements at 150 °C, the average response was 38%.The responses of the PPy/MoS2 sensor to other gasses at 50 ppm or higher concentrations at 150 °C are shown in Figure 9b.The responses of the PPy/MoS2 sensor to interferent gasses, such as ammonia (NH3), sulfur dioxide (SO2), hydrogen sulfide (H2S), and nitric oxide (NO), were lower than the response to NO2.These results demonstrate the excellent selectivity and repeatability of the PPy/MoS2 composite sensor.As shown in Table 1, the PPy/MoS 2 composite thin-film sensor prepared in this work has a lower operating temperature than traditional metal oxide sensors, and it has a faster response/recovery time than pure conductive polymer thin-film sensors.Figure 9a shows the repeatability of the PPy/MoS 2 sensor measurements for NO 2 gas at 100 ppm, where it is apparent that the response and recovery characteristics were maintained without significant degradation.Over six cycles of measurements at 150 • C, the average response was 38%.The responses of the PPy/MoS 2 sensor to other gasses at 50 ppm or higher concentrations at 150 • C are shown in Figure 9b.The responses of the PPy/MoS 2 sensor to interferent gasses, such as ammonia (NH 3 ), sulfur dioxide (SO 2 ), hydrogen sulfide (H 2 S), and nitric oxide (NO), were lower than the response to NO 2 .These results demonstrate the excellent selectivity and repeatability of the PPy/MoS 2 composite sensor.The results of the long-term stability tests are shown in Figure 10.The resistance the PPy/MoS2 sensor was investigated over 40 days of exposure to a fixed NO2 concent tion of 100 ppm (measured at 5-day intervals) at the operating temperature.The PPy/M sensor initially showed a maximum response of 38.5%, which gradually decreased to 2 and eventually stabilized, with no further decrease in the sensitivity over time after days; the overall decrease in sensitivity was approximately 25%.After 30 days, the sen tivity no longer decreased with time, and there was an overall decrease of approximat 25%.In contrast, the sensitivity of the pure PPy sensor decreased by approximately 6 after 30 days.Thus, the introduction of MoS2 nanosheets can improve the long-term s bility of the device to some extent.Furthermore, the high reproducibility of the response of the PPy/MoS2 sensor to 100 ppm NO2 indicates that the MoS2-doped PPy g sensor has good potential.In addition, the performance of the sensor is also affected environmental humidity and the film bending angle, as shown in Supplementary Do ments S4 and S5.PPy is a p-type semiconductor whose principal charge carriers are holes, and wh the oxidizing gas NO2 passes over the sensor, the electrons in the polymer are trapped form the anion NO2 − , and hence, the number of holes in the material increases, which lea The results of the long-term stability tests are shown in Figure 10.The resistance of the PPy/MoS 2 sensor was investigated over 40 days of exposure to a fixed NO 2 concentration of 100 ppm (measured at 5-day intervals) at the operating temperature.The PPy/MoS 2 sensor initially showed a maximum response of 38.5%, which gradually decreased to 29% and eventually stabilized, with no further decrease in the sensitivity over time after 30 days; the overall decrease in sensitivity was approximately 25%.After 30 days, the sensitivity no longer decreased with time, and there was an overall decrease of approximately 25%.In contrast, the sensitivity of the pure PPy sensor decreased by approximately 67% after 30 days.Thus, the introduction of MoS 2 nanosheets can improve the long-term stability of the device to some extent.Furthermore, the high reproducibility of the re-response of the PPy/MoS 2 sensor to 100 ppm NO 2 indicates that the MoS 2 -doped PPy gas sensor has good potential.In addition, the performance of the sensor is also affected by environmental humidity and the film bending angle, as shown in Supplementary Figures S4 and S5.The results of the long-term stability tests are shown in Figure 10.The resistance of the PPy/MoS2 sensor was investigated over 40 days of exposure to a fixed NO2 concentration of 100 ppm (measured at 5-day intervals) at the operating temperature.The PPy/MoS2 sensor initially showed a maximum response of 38.5%, which gradually decreased to 29% and eventually stabilized, with no further decrease in the sensitivity over time after 30 days; the overall decrease in sensitivity was approximately 25%.After 30 days, the sensitivity no longer decreased with time, and there was an overall decrease of approximately 25%.In contrast, the sensitivity of the pure PPy sensor decreased by approximately 67% after 30 days.Thus, the introduction of MoS2 nanosheets can improve the long-term stability of the device to some extent.Furthermore, the high reproducibility of the re-response of the PPy/MoS2 sensor to 100 ppm NO2 indicates that the MoS2-doped PPy gas sensor has good potential.In addition, the performance of the sensor is also affected by environmental humidity and the film bending angle, as shown in Supplementary Documents S4 and S5.PPy is a p-type semiconductor whose principal charge carriers are holes, and when the oxidizing gas NO2 passes over the sensor, the electrons in the polymer are trapped to form the anion NO2 − , and hence, the number of holes in the material increases, which leads gives up its trapped electrons, and electron-hole recombination occurs.The steps of the sensing mechanism are shown in the following equations [42]: PPy − e − ⇄ PPy + . ( Doping is a key parameter that plays an important role in the gas sensing mechanisms of chemoresistive gas sensors.In the PPy/MoS 2 hybrid nanocomposite sensor, a p-n junction formed between the p-type PPy and n-type MoS 2 .Upon exposure to oxidized NO 2 gas (electron acceptance), the resistance of the PPy/MoS 2 hybrid nanocomposite sensor decreased.Electron charge transfer is the main cause of the change in resistance in the PPy/MoS 2 hybrid nanocomposite sensors upon NO 2 gas adsorption.Upon exposure to NO 2 gas, electron charge transfer can occur between NO 2 and PPy because MoS 2 nanoparticles are embedded in the PPy matrix.The interaction of NO 2 gas molecules with the electronic network of PPy with embedded MoS 2 nanosheets leads to a decrease in the sensor resistance [20,[43][44][45].The reduced resistance of the sensor film confirms that the PPy/MoS 2 composite sensor behaves as a p-type semiconductor and that the charge transfer is mainly due to PPy.A schematic energy diagram of heterojunction between PPy and MoS 2 is illustrated in Figure 11b.PPy behaves as a p-type semiconductor with a bandgap of 2.7 eV, and MoS 2 is an n-type semiconductor with a bandgap of 1.2 eV.When the p-type PPy and n-type MoS 2 are in contact, the mutual diffusion of the two main carriers at the interface results in the formation of a p-n heterojunction and a self-assembled depletion layer.The adsorption and desorption of NO 2 resulted in the modulation of the width of the depletion layer of the p-n heterojunction.When gaseous NO 2 interacts with the surface of the PPy/MoS 2 hybrid nanocomposite sensor, the width of the depletion layer decreases owing to the decrease in the sensor resistance, resulting in easier transport of the charge carriers in the corresponding energy band of the sensor [46][47][48].As shown in the SEM images, the MoS 2 nanosheets were wrapped within and interconnected with the PPy, providing electron transfer pathways during gas sensing.In addition, the introduction of 2D MoS 2 nanosheets increased the dispersion of PPy, and the looser and more porous structure of the binary composite material had a larger surface area, which greatly facilitated gas diffusion, improving the performance of the sensor.
Polymers 2024, 16, x FOR PEER REVIEW 10 of 13 to a decrease in the electrical resistance.When air passes over the sensor, the absorbed NO2 − gives up its trapped electrons, and electron-hole recombination occurs.The steps of the sensing mechanism are shown in the following equations [42]: PPy e ⇄ PPy .
Doping is a key parameter that plays an important role in the gas sensing mechanisms of chemoresistive gas sensors.In the PPy/MoS2 hybrid nanocomposite sensor, a pn junction formed between the p-type PPy and n-type MoS2.Upon exposure to oxidized NO2 gas (electron acceptance), the resistance of the PPy/MoS2 hybrid nanocomposite sensor decreased.Electron charge transfer is the main cause of the change in resistance in the PPy/MoS2 hybrid nanocomposite sensors upon NO2 gas adsorption.Upon exposure to NO2 gas, electron charge transfer can occur between NO2 and PPy because MoS2 nanoparticles are embedded in the PPy matrix.The interaction of NO2 gas molecules with the electronic network of PPy with embedded MoS2 nanosheets leads to a decrease in the sensor resistance [20,[43][44][45].The reduced resistance of the sensor film confirms that the PPy/MoS2 composite sensor behaves as a p-type semiconductor and that the charge transfer is mainly due to PPy.A schematic energy diagram of the heterojunction between PPy and MoS2 is illustrated in Figure 11b.PPy behaves as a p-type semiconductor with a bandgap of 2.7 eV, and MoS2 is an n-type semiconductor with a bandgap of 1.2 eV.When the p-type PPy and n-type MoS2 are in contact, the mutual diffusion of the two main carriers at the interface results in the formation of a p-n heterojunction and a self-assembled depletion layer.The adsorption and desorption of NO2 resulted in the modulation of the width of the depletion layer of the p-n heterojunction.When gaseous NO2 interacts with the surface of the PPy/MoS2 hybrid nanocomposite sensor, the width of the depletion layer decreases owing to the decrease in the sensor resistance, resulting in easier transport of the charge carriers in the corresponding energy band of the sensor [46][47][48].As shown in the SEM images, the MoS2 nanosheets were wrapped within and interconnected with the PPy, providing electron transfer pathways during gas sensing.In addition, the introduction of 2D MoS2 nanosheets increased the dispersion of PPy, and the looser and more porous structure of the binary composite material had a larger surface area, which greatly facilitated gas diffusion, improving the performance of the sensor.

Conclusions
In this study, PPy/MoS2 binary composite materials were synthesized via chemical polymerization and mechanical blending, and their morphologies and structures were characterized using techniques including XRD, FTIR, and SEM.A flexible NO2 sensor was then fabricated from the composite material on a polyimide substrate using a flexible electronic printer.At room temperature, in an atmosphere of 100 ppm NO2, the sensitivity of the 10% PPy/MoS2 composite sensor was higher than that of an analogous sensor made

Conclusions
In this study, PPy/MoS 2 binary composite materials were synthesized via chemical polymerization and mechanical blending, and their morphologies and structures were characterized using techniques including XRD, FTIR, and SEM.A flexible NO 2 sensor was then fabricated from the composite material on a polyimide substrate using a flexible electronic printer.At room temperature, in an atmosphere of 100 ppm NO 2 , the sensitivity of the 10% PPy/MoS 2 composite sensor was higher than that of an analogous sensor made using pure PPy (49.8% vs. 5%).However, the response and recovery times of the two sensors at room temperature were greater, and the response and recovery times of the two sensors were also greater at higher temperatures.The optimal temperature, in terms of the response and recovery times, was 150 • C, with the response time of the 10% PPy/MoS 2 sensor being shortened from 572 s (at room temperature) to 120 s and the recovery time being shortened from 461 s (at room temperature) to 57 s.In addition, the sensor still retained relatively good resolution over low concentrations of 1-20 ppm, with a sensitivity of 8% to 1 ppm NO 2 for the 10% PPy/MoS 2 sensor at 150 • C. The enhanced response of the composite materials is attributed to the formation of p-n heterostructures and self-assembled depletion layer between the p-type PPy and n-type MoS 2 .Doping with MoS 2 resulted in a larger surface area and more contact sites between the NO 2 gas and the material.In addition, the introduction of a heating layer resulted in a synergistic enhancement in the sensor sensitivity and response/recovery speed.Therefore, it was demonstrated that the composite material fabricated by doping PPy with MoS 2 is an effective NO 2 gas sensing material that provides a foundation for the future development of flexible, wearable trace gas detection devices.

Figure 2 .
Figure 2. Schematics of (a) overall sensor structure, (b) multilayer sensor structure, and (c) gas sensing test platform.

Figure 2 .
Figure 2. Schematics of (a) overall sensor structure, (b) multilayer sensor structure, and (c) gas sensing test platform.

Figure 2 .
Figure 2. Schematics of (a) overall sensor structure, (b) multilayer sensor structure, and (c) gas sensing test platform.

Figure 3 .
Figure 3. SEM images of (a,b) the PPy and (c-f) the PPy/MoS2 composite, and (f1-f4) elemental distribution maps of the view of the composite shown in the SEM image in panel (f).

Figure 3 .
Figure 3. SEM images of (a,b) the PPy and (c-f) the PPy/MoS 2 composite, and (f1-f4) elemental distribution maps of the view of the composite shown in the SEM image in panel (f).

Figure 5 .
Figure 5.The TG and DTA curves of (a) PPy and (b) the PPy/MoS2 composite.

Figure 4 .
Figure 4. (a) XRD patterns and (b) FTIR spectra of pure PPy and PPy/MoS 2 composites with various MoS 2 mass fractions.
are the TG and DTA curves, respectively.The PPy sample continuously lost weight during heating from the start (at 100 • C).The thermal decomposition of PPy can be divided into two stages: (1) the first stage of weight loss occurred at the range of 300-420 • C and resulted from the detachment of water molecules from PPy; (2) the weight loss between 420 and 650 • C was a result of the degradation of the PPy macromolecular chains.The polymer completely decomposed above 650 • C. The TG and DTA curves remained constant at temperatures above 650 • C, which indicates that the decomposition of PPy began at 300 • C [37]. Figure 5b shows the TG and DTA curves of the PPy/MoS 2 composite.These have similar shapes to those of PPy, with two relatively symmetric and distinct heat absorption peaks at the ranges of 350-400 and 400-530 • C. The composite material lost very little weight below 250 • C. When compared with that of pure PPy, the thermal stability of PPy in the hybrid material is almost unchanged, and the TG analysis indicates that that the maximum operating temperature of the sensor can be set to 350 • C. Polymers 2024, 16, x FOR PEER REVIEW 6

Figure 5 .
Figure 5.The TG and DTA curves of (a) PPy and (b) the PPy/MoS2 composite.Figure 5.The TG and DTA curves of (a) PPy and (b) the PPy/MoS 2 composite.

Figure 5 .
Figure 5.The TG and DTA curves of (a) PPy and (b) the PPy/MoS2 composite.Figure 5.The TG and DTA curves of (a) PPy and (b) the PPy/MoS 2 composite.

Figure 6 .
Figure 6.(a) Resistance and (b) sensitivity of sensors based on composites with different MoS2 tents at 100 ppm NO2.

Figure 6 .
Figure 6.(a) Resistance and (b) sensitivity of sensors based on composites with different MoS 2 contents at 100 ppm NO 2 .

Figure 7 .
Figure 7.Comparison of (a) sensitivity and (b) response/recovery times of 10% PPy/MoS 2 sensor at 100 ppm NO 2 and different temperatures.

Figure 8 .
Figure 8.The change in resistance with a NO2 concentration in the range of 1 to 20 ppm at 150 °C for the (a) pure PPy and (b) PPy/MoS2 10% sensors.

Figure 8 .
Figure 8.The change in resistance with a NO 2 concentration in the range of 1 to 20 ppm at 150 • C for the (a) pure PPy and (b) PPy/MoS 2 10% sensors.

Figure 10 .
Figure 10.The long-term stabilities of the PPy and PPy/MoS2 sensors.

Figure 10 .
Figure 10.The long-term stabilities of the PPy and PPy/MoS2 sensors.

Figure 10 .−
Figure 10.The long-term stabilities of the PPy and PPy/MoS 2 sensors.PPy is a p-type semiconductor whose principal charge carriers are holes, and when the oxidizing gas NO 2 passes over the sensor, the electrons in the polymer are trapped to form the anion NO 2 − , and hence, the number of holes in the material increases, which leads to a decrease in the electrical resistance.When air passes over the sensor, the absorbed NO 2 −

Figure 11 .
Figure 11.(a) A schematic diagram of the mechanism underlying the enhanced performance of the PPy/MoS2 composite sensor.(b) A schematic energy diagram of the PPy-MoS2 heterojunction.

Figure 11 .
Figure 11.(a) A schematic diagram of the mechanism underlying the enhanced performance of the PPy/MoS 2 composite sensor.(b) A schematic energy diagram of the PPy-MoS 2 heterojunction.
Author Contributions: Conceptualization, Y.S. and Y.H.; methodology, B.T.; software, M.C. and C.Z.; investigation, M.C. and Q.C.; data curation, M.C.; writing-original draft preparation, K.Z.; writing-review and editing, K.Z.All authors have read and agreed to the published version of the manuscript.Funding: This research was funded by a grant from the National Natural Science Foundation of China, grant number 62271176, and the China National Basic Enhancement Program, 2022-JCJQ-JJ-0438.

Table 1 .
Performance parameters and operating temperatures of several common types of NO2 gas sensors.

Table 1 .
Performance parameters and operating temperatures of several common types of NO 2 gas sensors.